In this description, the terms “P red” and “red phosphorus” may be used interchangeably. Also the terms “P white” and “white phosphorus” may be used interchangeably.
A phosphorus effusion cell is used for generating a molecular beam of phosphorus (P2) in a molecular beam epitaxy (MBE) reactor. The cell uses red phosphorus (P-red) as the source material. The P-red is first converted into white phosphorus (P-white). The P-white is then heated to produce phosphorus (P4), which is directed through a control valve to a thermal cracker. The thermal cracker cracks P4 molecules into P2 molecules. The use of P2 instead of P4 for the growth of indium phosphide and related materials is advantageous because of two reasons, namely material quality and safety. A control valve between the white phosphorus container and the thermal cracker is used to control the P2 flux.
The phosphorus charge used in MBE is in the form of solid red phosphorus. In this description, P-red means red allotrope of phosphorus and P-white means white allotrope of phosphorus.
As is well known in the art, when P-red is heated in vacuum to over 300° C. it creates (by sublimation) P4 molecules. The P4 pressure of P-red at 300° C. is 8.5 mbar and the P2 pressure of P-red at 300° C. is less than 4×10−5 mbar. Therefore practically all phosphorus sublimated from P-red is in P4 molecular form. When P4 is condensed on a colder surface it will condense as P-white and the P-white pressure at 300° C. is very high, about 1000 mbar. Further, the P4 molecules can be thermally cracked to P2 and when P2 condenses on cold surface it will condense as P-red. It is also known that P-white is highly flammable at room temperature and therefore the devices are to be kept under vacuum.
Several different arrangements are known for phosphorus effusion sources. For example, in the patent U.S. Pat. No. 5,431,735 a device is proposed, that comprises a vessel in which vapour of phosphorus is produced from red phosphorus (P-red). The vessel consists of two concentric zones having different temperatures. One zone is used for sublimation of P-red and the other for condensation and re-evaporation of P-white. P-red is continuously heated during operation and P-white is continuously condensed into said condenser zone. The phosphorus flows continuously from the P-red container, through the P-white container and further to the thermal cracker. In this device, the P-red source material must be continuously heated during operation in order to maintain the flow from P-red to P-white and further to the thermal cracker.
The patent U.S. Pat. No. 6,030,458 discloses a device in which the P-red and P-white containers are enclosed by a vacuum jacket. The two containers are connected to each other with a tube. The device is operated in a mode where P-red is first converted into P-white and after conversion P-white is evaporated to form P4. After conversion the P-red container is kept at an elevated temperature to prevent re-condensation of phosphorus into the P-red container. In this device, P-red must be continuously heated in order to prevent re-condensation of P4 into the P-red container.
The Applicants have further found that the prior art devices have also the following problems.
In prior art devices the two containers are physically connected, and therefore it is difficult to achieve stable P4 pressure while two different temperature zones are connected to each other. Thus, a change in one container temperature will affect the temperature in the other container.
On the other hand, production scale MBE systems require large capacity P-red charges to be used in order to increase the time interval between the system source fillings, which normally requires venting of the MBE growth chamber. As in both above-mentioned devices the P-red charge must be kept continuously at elevated temperatures, the response time to temperature changes becomes very long due to the large thermal mass of the P-red charge.
Moreover, as described in the patent U.S. Pat. No. 5,431,735, any internal hot part in the effusion cell can cause thermal cracking of P4 into P2 inside the cell. Typically, the P-red heater element must be run at a higher temperature than the desired P-red temperature because of thermal leaks. This results in a small fraction of P4 to crack into P2 in the heater element or on the hot walls of the P-red container before the P4 flux reaches the cracker zone. Because P2 condenses as P-red, some condensation of P-red occurs inside the effusion cell in the prior art devices. Over time this results in accumulation of red phosphorus deposits inside the P-white container. This has a detrimental effect on the film quality and the thermal behaviour of the P-white container.
Last but not least, in the both prior art devices described above, the P-white container is exposed to air during source refilling with P-red. This may be detrimental to the purity of the grown epitaxial layers as well as to safety, as P-white is highly flammable at room temperature.